This achievement can be aquired in Chapter 7. By this point you should have easily built your fighting skill up to "Untouchable". If not, hit the weights or the heavy bag in the main yard to do so before attempting the achievement. Once your fighting skill is maxed, find Mr. King in the yard and he will set you up with an underground fight. Choose the first opponent, Soda Pop. Choosing Soda Pop will allow you to complete this achievement rather quickly and easily.
There are a couple of ways to go about defeating Soda Pop, and depending on which difficulty setting you are playing through, this could net you some other achievements in the process:
1) Unleash a flurry of quick punches by repeatedly tapping the "A" button. Continue this throughout the fight. With Soda Pop's lack of speed, he will almost never hit you.
2) Wait until you see Soda Pop about to throw a punch and tap the "Right Bumper" to counter his punch. As this happens, tap the corresponding "quick-time" button that appears on the screen. Repeat this until his health is depleted. Again, you can very easily defeat him in seconds with getting a scratch using this method .
When his health is low enough, you will be prompted on the screen to tap "Y". This will perform a finishing move and "BAM", the fight is over!
Either way you go, be sure to kick your opponent when he's down (literally!). To do this, walk up next to him when he is on the ground and tap "A". You should be able to get one or two kicks in before he gets back to his feet. These do a tremendous amount of damage to the opponent!
Just repeat this 50 times to unlock the "Ringer" achievement.